Our British Pat. No. 2 085 474 describes a cell for electrophoretically coating a can. The cell comprises a hollow body which surrounds the can, a lid at the top which closes the cell, and a hollow mandrel disposed within the cell and so arranged that the can is held mouth downwards in equi-spaced relationship to the interior of the cell and the exterior of the mandrel. With an electrolyte, in the form of an electrophoretic coating material, passing through the mandrel into the can, an electric potential difference is applied between the can and the mandrel to deposit a coating on the interior of the can.
Our British Pat. No. 2 085 922 describes an apparatus for applying a series of electric current pulses between an electro-coating cell and a can enclosed within that cell whereby to progressively electrophoretically coat the can. The apparatus includes a rotating turntable, a plurality of cells disposed around the circumference of the turntable, and a pair of segmented sliprings carried by the turntable. Each cell is electrically connected between corresponding segments of the two slip-rings. Respective sets of stationary brushes cooperate with the two slip-rings at circumferentially spaced positions. Each brush set supplies an electric current pulse of predetermined time duration to each cell in turn as the turntable rotates. Thus, as the turntable rotates, and each cell is carried past the successive sets of brushes, that cell is energised by a succession of fixed duration current pulses.
In order to prevent arcing between adjacent slip-ring segments, adjacent segments are spaced apart by spacers, and each current pulse is timed so as to start only after a slip-ring segment is in full contact with a brush set, and to continue for a predetermined time period such that current flow always ceases before full contact of the brush set with the segment ceases.
As the duration of each current pulse is fixed, the proportion of the slip-ring segment travel from one brush set to the next during which each such current pulse flows depends on the rate of travel of the slip-rings past the brush sets, the maximum proportion being obtained when the turntable speed is highest. Thus, the maximum turntable speed determines the duration of the current pulses, and the time periods which the electro-coating process is temporarily halted are short.
On the other hand, at low turntable speeds, current flows during only a small proportion of that slip-ring segment travel. Consequently, at low turntable speeds there are considerable time periods between successive current pulses, during which the electro-coating process is halted.
It has been observed by the present inventors (a) that during the inter-pulse periods where the electro-coating process is temporarily halted, the electrical resistance of the coating so far deposited increases with time, so that for a given applied voltage the rate of depositing additional coating material during the next pulse is reduced, and (b) that with that prior art apparatus, a desired coating weight is achieved more quickly when the turntable speed is higher than when the speed is lower.
The present invention seeks to provide an apparatus in which (a) each current pulse flows for as long a time as possible regardless of the turntable speed, (b) the time periods during which the electro-coating process is temporarily halted are minimised, and (c) the speed of the electro-coating apparatus can be readily adjusted so as to match it to the speed of other machines in a can production line without adversely affecting the performance of the electro-coating process in each cell.